Generally, pipes are roughly classified into welding pipes and seamless pipes. An electric resistance welding pipe as one of the welding pipes is manufactured by rounding a sheet by roll forming or the like, and then confronting each edge and welding. In the welding pipes, toughness and strength of weld are generally bad compared with those of a mother strip. Thus, in use of the pipes, it is an inevitable problem to ensure certain toughness and strength of weld for each application.
For example, since a line pipe for transporting crude oil or natural gas is often laid in the cold latitudes, low temperature toughness is essentially concerned. Moreover, strength is importantly concerned with casings for protecting mining pipes in oil wells for mining crude oil.
Among the welding pipes, the electric resistance welding pipe is formed by welding without using weld metal, therefore a configuration of manufacturing equipment thereof includes at least an apparatus for supplying a strip, apparatus for reforming a shape of the supplied strip, apparatus for roll-forming the reformed strip, apparatus for inductively heating lateral edges of the roll-formed strip, and apparatus for pressure-welding the inductively heated, lateral edges. Furthermore, the manufacturing equipment often includes an apparatus for performing heat treatment of weld of the welded pipe, and an apparatus for shaping a profile of the pipe.
The apparatus for roll-forming the strip typically includes a break-down roll stand for arcuately bending a strip, cage roll stand for rounding the strip being arcuately bent into a pipe shape, and finpass forming stand for shaping the pipe that has been rounded.
Such conventionally used, manufacturing equipment of electric resistance welding pipes is shown in FIG. 7. The manufacturing equipment is configured by an uncoiler 1 for supplying a strip 20; a leveler 2 for reforming the strip 20 to be flat; a roll forming machine 5 for gradually rounding the reformed strip 20; an induction heater 6 for inductively heating right and left, two lateral edges of the rounded strip 20; a squeeze roll (electric resistance welding section) 7 for contacting by pressing the inductively-heated lateral edges of the strip so as to be formed into a pipe 21; a bead cutter 8 for cutting a weld bead portion of the pipe 21; a sizer 9 for adjusting outer diameter of the pipe 21 after cutting; and a pipe cutter 10 for cutting out the pipe 21 being adjusted in outer diameter into a predetermined length. The roll forming machine 5 has a predetermined number (herein, two) of finpass forming stands 4 in a last stage. A fin shape of the finpass forming stand includes a one-stage tapering shape.
Typically, a hot-rolled sheet to be a mother strip of a pipe is subjected to composition design or heat treatment in consideration of properties of the mother strip after being formed into the pipe to secure properties of the mother strip such as toughness and strength.
However, since characterization of welded seam is severely affected by a welding method rather than the composition design or heat treatment of the mother strip, development in welding technique is particularly important in the case of electric resistance welding.
As a cause of problems for electric resistance welding, oxides called penetrators are given, which are generated on a lateral edge of a strip to be welded. In many cases, the penetrators are not discharged from the edge along with melting steel during electric resistance welding and remained thereon, and the remained penetrators cause reduction in toughness, leading to insufficient strength.
Thus, to remove the penetrators from weld as a main cause of the detects in electric resistance welding, earnest investigation has been made so far on an active discharging technique of melting steel from lateral edges of a strip to be welded.
For example, JP-A-2001-170779 or JP-A-2003-164909 describes an example of investigation on a shape of each lateral edge of a strip to be welded. That is, in the example, it is intended that right and left, lateral edges of a strip to be welded, which are typically formed in an approximately rectangular shape by slitting or edge shaving, are tapered before roll forming, and the tapered shape of the lateral edges improves discharge of melting steel during electric resistance welding.
For example, JP-A-57-031485 or JP-A-63-317212 describes an example of investigation on a shape of each lateral edge of a strip. In the example, it is intended that each lateral edge of a strip, which is typically formed in an approximately rectangular shape by slitting or edge shaving, is processed in shape before electric resistance welding so that the processed shape of the lateral edge improves discharge of melting steel during welding.
However, in the conventional methods described in JP-A-2001-170779 or JP-A-2003-164909, since a hole-shape rolling roll, cutting bite, and shaving roll are introduced as tapering means by a simply listed manner, there have been various problems for specifically using the method for a manufacturing process of electric resistance welding pipes, and therefore further detailed investigation has been needed.
That is, strips having various kinds of thickness are formed into electric resistance welding pipes in an actual manufacturing process of electric resistance welding pipes, and, for example, when the hole-shape rolling roll is used to shape the tapering on right and left, two lateral edges of a strip, different hole-shape rolling rolls must be prepared for each kind of strips having different thickness and the hole-shape rolling roll must be changed for each kind of the strips, therefore manufacturing efficiency of the pipes has been problematically reduced. Moreover, when the lateral edges of the strip are shaped with tapering by the cutting bite or the shaving roll before roll forming, since most of the tapering is crushed by finpass rolling using finpass forming stand, it has been significantly difficult that the lateral edges are shaped with the tapering by the cutting bite or the shaving roll in consideration of such crushing.
We investigated the method described in JP-A-57-031485 and, as a result, it was clarified that even if the amount of upset in finpass forming was greatly changed, it was significantly difficult to contact only a part of each lateral edge of the strip 20 to the fin of the finpass hole-shape roll. This is because since the lateral edge of the strip 20 was slightly work-hardened in a previous forming process, the whole lateral edge of the strip is easily deformed along the fin so as to perfectly fill the fin portion, consequently a shape of the fin is printed to the lateral edge of the strip. As a result, the lateral edge of the strip 20 is not in a desired shape immediately before electric resistance welding.
Moreover, we investigated the method described in JP-A-63-317212 and, as a result, confirmed the following. That is, to shape the tapering on the whole lateral edge of the strip 20 using the edger roll 11 in the middle of roll forming (at the upstream side of the finpass forming stand), since the edger roll, of which the diameter is gradually increased from a pipe outer-surface side to a pipe inner-surface side, needs to be used for forming as described in JP-A-63-317212, a lateral edge to be an edge at the pipe inner-surface side is shaved by the edger roll, which may problematically induce pads called “whisker.” Furthermore, since large reaction force that opens the pipe-shaped strip 20 outward is exerted in a cross section direction of the strip 20 to be roll-formed, pressure between the edger roll 11 and the lateral edge of the strip 20 is necessarily reduced. As a result, as in JP-A-57-031485, the strip is hardly work-hardened through reduction of the lateral edge by the edger roll, and even if the amount of upset is reduced in subsequent finpass forming, the strip substantially fills the fin portion, therefore it is difficult that the lateral edge of the strip 20 is shaped as described in JP-A-63-317212.
It could therefore be advantageous to provide manufacturing equipment of electric resistance welding pipes, in which a lateral edge shape can be made into an appropriate shape immediately before electric resistance welding, thereby melting steel is sufficiently discharged during electric resistance welding so that penetrators are securely removed, consequently an electric resistance welding pipe having excellent characterization of welded seam can be obtained.
It could also be advantageous to provide manufacturing equipment of electric resistance welding pipes having excellent characterization of welded seam, in which each lateral edge of a strip is shaped with appropriate tapering immediately before electric resistance welding, so that welding quality can be kept excellent, in addition, reduction in manufacturing efficiency can be suppressed.